Scanning electron microscopes (SEM) are very useful for imaging very small elements on a sub-micron scale, with a resolution on the order of nanometers. Therefore, various SEM systems are used in the semiconductor industry for engineering and metrology. Recently, much attention has also been given to the use of SEM in investigation of defects on semiconductor circuits. Since the size of defects of interest (i.e., killer defects) continues to shrink with the shrinking of the design rules, there's a continuous need for improvement in the images obtained by such SEM.
As is well known to those skilled in the art, SEM images are obtained by directing a primary electron beam onto a sample, and using detectors to collect electrons returned from the sample. Some of these electrons are back scattered electrons (BSE) reflected as a result of elastic collisions. Others are secondary electrons (SE) emitted from the sample as a result of inelastic collisions. For better understanding of the discussion that follows, the reader's attention is directed to FIG. 1, depicting the energy spectrum of electrons emitted from a sample upon the impingement of a primary electron beam (the plot is adopted from Image Formation in Low-Voltage Scanning Electron Microscopy, L. Reimer, SPIE, Vol. TT12). It is conventionally accepted that collected electrons having energy up to 50 eV are SE, while collected electrons having energy above 50 eV are BSE.
SEM images have been traditionally provided as gray scale. However, for some time there has been an effort to provide color SEM images. Two basic methods have been employed for applying color to SEM images. The first method is based on the fact that the human eye is more sensitive to color variation than to shades of gray. Therefore, the gray scale was used to modulate color and it is sometimes referred to as color modulation. Another method was developed basically to better distinguish between features in two different signals (for example, one of SE and one of BSE). Thus, each signal was color coded and the resulting signals combined to obtain a color picture. The journal SCANNING has dedicated an entire issue to color SEM and much information about these systems is disclosed there--SCANNING, Vol 3, 3, (1980). One article in that issue, COLOR CONVERSION IN ELECTRON MICROSCOPY, by A. V. Crewe, aptly summarizes the advantages of color in SEM and the various techniques used for color SEM images.
As is known in the art, and as can be understood from the above noted works, SEM images are generally created from SE or BSE depending on the purpose of the study. That is, when the study requires the ability to distinguish between different materials in the sample, BSE detectors are used. See, for example, COLOUR ENCODING OF VIDEO SIGNALS IN SEM, E. I. Rau, et al., Id. On the other hand, when it is important to understand the topography of the sample, SE detectors are used. See, for example, U.S. Pat. No. 5,212,383 describing color modulation of wide energy band SE signals. Moreover, by assigning a certain color to a SE detector and another color to a BSE detector and then combing the images, one can obtain material and topography information in the same image. See, for example, A METHOD FOR PREPARING COLORED SCANNING ELECTRON MICROGRAPHS USING SE AND BSE IMAGES, K. Tanaka, Id.
However, in spite of the above efforts, there are still particular deficiencies of SEM images that are not addressed by these systems. These deficiencies are of particular interest to the semiconductor industry. One particular issue of importance to the semiconductor industry is a clear delineation of edges of features. For example, when one takes a SEM image of a bit line, it is important that the edges of the bit line be clearly displayed so that the edges can be investigated. Similarly, when one takes a SEM image of a defect, it is important to clearly distinguish the edges of the defect so that its boundaries can be determined.
Another issue of interest is clear contrast of materials. Specifically, theoretical and experimental works showed the capabilities of using BSE for distinguishing between heavy elements, but BSE lacked this capability for the light elements. Thus, a better method is needed for distinguishing between elements, especially when similar, light, elements are present within the investigated sample.
Voltage Contrast is a common way to test the electrical connectivity of features in the semiconductor industry. In one of the Modes of Voltage Contrast-Passive Voltage Contrast (PVC)--all the conductors that are grounded yield different signal from those that are not grounded. This happens because those conductors that are not grounded and float on the insulator are charged and they usually exhibits higher emission yield. There is a need to be able to enhance and diminish this effect on the resulting image, as needed for particular applications.